When an over-hang length for a tool to stick out is long with respect to a drill diameter, the rigidity of the tool will decrease. Therefore, a tool deflection occurs during a drilling process, resulting in a deterioration of the straightness of a drilled hole or a breakage of the drill. When using a drill for deep hole drilling, usually the flute length of the drill is made shorter than the depth of the drilled hole, thereby drilling a hole by means of “step feed”. Here, so-called “step feed” in a drilling process is a method of drilling a hole by repeatedly moving the drill forward and backward with respect to the hole in a step-by-step movement.
In patent document 1 (Japanese Unexamined Patent Application Publication No. 2003-266223), as a small-diameter drill for deep hole drilling suitable for use in step feed, there has been disclosed a drill in which the flute is shortened to increase mechanical strength, so that it can be used for drilling deep holes having a depth of 10 or more times the drill diameter in an extremely hard material having a hardness of HRC 45 or more.
In patent document 2 (Japanese Unexamined Patent Application Publication No. H8-71824), a drill is disclosed which comprises: a back tapered portion with its diameter reducing from the front point of the drill, a small-diameter portion having a diameter smaller than the drill diameter, and a constant diameter portion having the same diameter as the drill diameter.
In patent document 3 (Japanese Unexamined Patent Application Publication No. H4-348803), a hole drilling method is disclosed in which after a guide hole has been formed and when a working drill gets into contact with the temporary hole, an in-plane component force acting on the drill is detected, and the position of the tip of the drill is moved in a direction in which the component force decreases.
In patent document 4 (Japanese Unexamined Patent Application Publication No. H6-134648), a cutting and drilling method is disclosed which uses a ball end mill to drill a hole by providing a depth of cut and a relative feeding movement in the X-axis and Y-axis directions to the ball end mill and work piece so that cutting is performed in a spiral and down-cut manner.
In patent document 5 (Japanese Unexamined Patent Application Publication No. 2003-260611), a boring method for boring an inclined hole in a plate material is disclosed, in which when an inclined hole is to be drilled in a plate material, a hemispherical hole is at first drilled for positioning the hole, then a drilling jig for hole drilling is obliquely attached to the plate material so that an inclined hole can be drilled in the plate material.
Patent document 1: Japanese Unexamined Patent Application Publication 2003-266223
Patent document 2: Japanese Unexamined Patent Application Publication H08-71824
Patent document 3: Japanese Unexamined Patent Application Publication H04-348803
Patent document 4: Japanese Unexamined Patent Application Publication H06-134648
Patent document 5: Japanese Unexamined Patent Application Publication 2003-260611